1. Field of the Invention
The disclosed invention is a controlled exhaust brake for an engine and method for maintaining a set pressure in the exhaust manifold. The claimed invention provides for a controlled exhaust brake having a controller that adaptively responds to changes in exhaust pressure relative to a set pressure. The controlled exhaust brake enhances engine retarding significantly. Exhaust pressure for optimum braking power is maintained over the full range of engine speed. Driver input is not required to maintain a high exhaust pressure. In addition, the claimed invention generates a backpressure load suitable for warming a cold engine after starting.
2. Description of Prior Art
It is well known in the art that the regulation of exhaust pressure in internal combustion engines assists in warming up the engine, and also provides for engine retarding power. It is also known in the art that engines operate with the greatest fuel efficiency and operating characteristics when an optimum operating temperature has been reached for the engine. When an engine operates below its optimum temperature during starting or warm-up of the engine, emissions of unburned fuel increase.
Light and medium duty diesel engines generally rely on exhaust brakes for engine retarding since a compression release type brake can easily develop loads that are too great for the smaller valve trains. The exhaust brake mechanism includes a restrictor element mounted in the exhaust system. Generally, exhaust gas generated by pistons of an engine is released into the exhaust system during an exhaust cycle. When the restrictor element is closed, backpressure resists the exit of gases during the exhaust cycle and retards the motion of the pistons, thereby providing an engine braking function. This system provides less braking power than a compression release engine brake, but also at less cost. Partial closing of the restrictor provides a light load against which the engine must work, causing the engine to warm up faster than it would without such restriction. This is particularly useful after starting a cold engine.
The amount of exhaust gas may vary depending on operating conditions, such as engine speed. When an engine is operated at lower engine speeds, less exhaust gas is available for providing backpressure, or engine retarding. When an engine is operated at higher speeds, more exhaust gas is available for providing backpressure. Engine speed may also vary due to load fluctuations on the vehicle engine, such as inclines and declines. In order to compensate for these fluctuations, some prior art systems use bypass valves, or waste-gates, to provide a means of controlling the exhaust gas flow. Other systems rely on driver input to adjust the position of the restrictor element. Repeated gear shifting has also been used to keep the engine speed and exhaust pressure high.
In many conventional exhaust brake systems, a certain amount of leakage by the exhaust restrictor is desirable so that when the restrictor is fully closed exhaust pressure will not rise above the system limit. This is generally accomplished by creating substantial clearance around the restrictor or by actually putting a hole through the exhaust restrictor element. In addition to leakage by the restrictor, in many systems the restriction is optimized to generate maximum allowable backpressure at rated engine speed. However, retarding power of exhaust brakes generally falls off sharply as engine speed decreases. Therefore, such conventional systems provide exhaust restriction that is too small to be effective at lower engine speeds given exhaust pressure decreases with engine speed. As such, they do not have the flexibility to optimize retarding power over the full engine speed range. Performance is improved considerably if exhaust pressure is maintained over the full speed range for engine retarding.
Attempts have been made to develop a system for optimizing the retarding power yield of exhaust brakes over the full speed range. In some systems, restriction is controlled by mechanical controls with hydraulic or pneumatic actuators. However, these systems can only control to a single exhaust set pressure. In addition, if the single set pressure is the maximum pressure allowed by the engine, the exhaust pressure will most likely exceed this limit pressure before it is brought back below the maximum pressure. Assured maximum pressure protection requires the set pressure to be less than the maximum allowed pressure.
In other systems, pressure relief mechanisms limit the maximum exhaust pressure. This approach may work at low engine speeds, but may hinder the ability to produce pressure at high engine speeds after the pressure relief has been activated. Therefore, such systems fail to optimize the retarding power for the full applicable engine speed range. Retarding power optimization with engine speed requires the set pressure to be variable with engine speed.
Another approach provides for a system that controls exhaust pressure to a maximum level permitted for the engine. In such a system, a signal pressure from the vehicle""s main air tanks is ported to a plunger. The plunger then extends a dish-valve to seal against the oncoming exhaust gas. The pressure balance between the area of the plunger and the dish-valve determines the resultant exhaust restriction.
One problem with past systems, as described above, is the inability to operate an exhaust brake at high exhaust pressure due to exhaust valve float. Valve float is the opening of the valve by means of gas pressure applied to the back face. Excessive valve float can cause high valve seating velocities, reduce the time the valve is seated and associated heat transfer and may cause contact of the valve with the piston. Further attempts to overcome this problem have been made, wherein a second valve is closed in the intake system. The pressure is thereby equalized across the valves and the valves are biased closed by the valve springs. A pressure relief valve is then placed in the system to establish a maximum exhaust pressure. The resultant system is overly complex, having various additional components.
Other systems use an external bypass to the main exhaust brake valve to create a variable exhaust brake. Exhaust gas pressure acts on a bypass valve and against a spring force resisting movement of the valve. Once the design limit for exhaust pressure is reached, the bypass opens. As engine speed increases, the bypass valve may open further to maintain pressure at the upper limit. Again, the resultant system is overly complex.
Therefore, utilization of bypass circuits to control exhaust pressure by directing a portion of the exhaust gas away from the main exhaust flow may partially control exhaust pressure, but at the cost of adding components and complexity to the system. Furthermore, many bypass circuits do not accurately control exhaust pressure given such circuits rely on control of biasing force of a plunger within the bypass circuit for regulating exhaust flow. Exhaust flow is regulated by determining the appropriate biasing force necessary for retarding in the bypass circuit. Adjustment of the bypass circuit plunger is based on this biasing force, instead of on the actual exhaust pressure. Therefore, exhaust pressure is controlled indirectly, which may lead to greater fluctuations of the desired optimal exhaust pressure, especially when engine speed fluctuates.
Even if retarding power is achieved over a range of engine speeds, the exhaust brake can stall the engine below a certain engine speed. In order to compensate for this problem, some systems provide for an electrical switch to monitor throttle position and a shift sensor in an automatic transmission. In such systems, exhaust braking is applied only when sufficiently high servo hydraulic pressure is attained after a downshift, thus preventing the engine from stalling. However, driver input is generally required in such systems.
Driver input has also been incorporated to establish dual use of the exhaust brake, functioning as both an engine retarder and an engine warm-up valve. For engine warm-up, the exhaust restrictor is only closed partially to provide a light load against which the engine can work. Heat energy is retained and the engine warms up faster. Other systems have used driver input to command restriction adjustment for turbocharger wastegating or variable geometry turbine settings. These vary the degree of cylinder charge and thereby the exhaust braking effort. Again, however, driver input is required in order to actuate such exhaust restriction, which may lead to less accurate or poorly timed adjustment.
Vehicle operators are not willing to run the engines at their rated speed to take advantage of maximum exhaust brake power. Therefore, there is a need for greater retarding power to be provided by the exhaust brake at engine speeds below rated. A simple orifice, designed to prevent over-pressure at high engine speed, does not provide the high exhaust pressure required at the lower engine speeds. Prior art has addressed the need to increase exhaust brake performance, but falls short of providing the flexibility to optimize retarding power over the full range of engine operating conditions.
It is apparent from the disclosures of the prior art that there remains a significant need for a practical method of and apparatus for regulating exhaust brake pressure in order to provide maximum effectiveness at all engine speeds. A practical system must control to a set pressure that is less than the maximum allowed pressure to avoid exceeding the upper limit. Also, a practical system requires the set pressure to be variable with engine speed so that retarding power can be optimized at the lower speeds. The present invention provides for a controlled exhaust brake that overcomes the deficiencies described above. In addition, the controlled exhaust brake of the present invention provides for minimum or no leakage when the restrictor is in the closed position, given the restrictor relies on computer control to maintain system exhaust pressure and prevent over-pressure.
A controlled exhaust brake for an engine comprises an exhaust restrictor located in an exhaust system downstream of an exhaust manifold of an engine. An actuator operably associated with the restrictor adjusts the restrictor. A pressure sensor upstream of the restrictor senses pressure in the exhaust system. A controller is in communication with the pressure sensor and the actuator, and determines a set pressure in the exhaust manifold correlated with speed of the engine. The controller then causes adjustment of the restrictor in order to achieve and maintain the set pressure in the exhaust manifold.
A temperature sensor may also be located upstream of the restrictor. The temperature sensor senses temperature upstream of the restrictor. The controller, also in communication with the temperature sensor, maintains a set temperature in the exhaust manifold. The set temperature is less than a maximum allowable temperature.
The claimed invention also relates to a controlled exhaust brake for an engine including a bypass valve operably associated with an exhaust manifold of an exhaust system of an engine. An exhaust restrictor located in the exhaust system downstream of the exhaust manifold is operably associated with the bypass valve. A first actuator is operably associated with the restrictor for opening or closing the restrictor so that when the restrictor is closed, an exhaust flow is channeled to the bypass valve. A second actuator is operably associated with the bypass valve for adjusting the bypass valve. A pressure sensor senses pressure in the exhaust manifold. A controller is in communication with the pressure sensor, the first actuator, and the second actuator, and determines a set pressure in the exhaust manifold correlated with speed of the engine. The controller then causes the restrictor to close, and causes adjustment of the bypass valve in order to maintain a set pressure in the exhaust manifold.
The claimed invention also relates to a method of regulating exhaust pressure in an engine, including the following steps: providing an exhaust restrictor located in an exhaust system downstream of an exhaust manifold of an engine; sensing pressure in the exhaust manifold; determining a set pressure in the exhaust manifold correlated with speed of the engine by a controller; and maintaining the set pressure in the exhaust manifold by causing adjustment of the restrictor by the controller.
The controlled exhaust brake of the claimed invention maintains a set pressure in an exhaust system in an engine, even with variable engine conditions. The claimed invention responds to changing pressure conditions, and thereby maintains the set pressure over the full engine speed range by adjusting the restrictor in the exhaust system. Control of the exhaust restrictor provides the ability to maintain an exhaust pressure required for optimal engine function. The restrictor may be placed at any point in the exhaust system, including upstream, downstream or integral to a turbocharger in a turbocharged engine system.
In the claimed invention, exhaust pressure is measured and compared to a desired set pressure, which is electronically stored in a controller. If the measured exhaust pressure equals the desired set pressure, the position of the exhaust restrictor is maintained. If the measured exhaust pressure does not equal the desired set pressure, the exhaust restrictor is adjusted, either opened or closed. This adjustment is controlled by the controller, and brings the exhaust pressure closer to the set pressure. The controller monitors both current pressure and past trends in pressure in order to determine the restrictor adjustment necessary to achieve the desired set pressure.
Specifically, a pressure signal is taken on the upstream side of the restrictor and is read by the controller at a pressure transducer. The controller is in communication with an actuator, which causes adjustment of the restrictor. The controller may also operate a solenoid mechanism to modulate the signal through a solenoid valve to the actuator. The actuator may be operated by air or oil, or may be electromagnetic. If power is lost to the controller and/or solenoid mechanism, the actuator moves the restrictor to the fully open position for failsafe operation.
As opposed to prior systems, simplicity of the claimed invention is maintained, given only a single exhaust restrictor is used in the exhaust system. Standard actuator hardware may be used to adjust the restrictor. The signal source to the actuator is modulated based on the prevailing exhaust pressure alone, or exhaust pressure and temperature together. Sensor input and stored target data constrain a control algorithm utilized by the controller to drive the system to converge for optimized exhaust brake performance at all applicable engine speeds.
It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only. The accompanying drawings, incorporated herein by reference, illustrate certain embodiments of the invention, and together with the detailed description serve to explain the principles of the present invention.